This series adds command line file descriptor passing support
via a new -add-fd option. This is a follow-on to the existing
QMP fd passing support provided in the following patch series:
comments.gmane.org/gmane.comp.emulators.qemu/165463
The new -add-fd option is designed to mirror the add-fd QMP
option as much as possible.

Before this can still be useful in libvirt, we need a way to pass in fds
for all of a backing chain, and not just the top-level. And you can't
argue that libvirt should be temporarily rewriting qcow2 metadata to
stick in names such as /dev/fdset/1 into the metadata. Consider the
case of:
/- guest1.img
base.img <- snap1.img <
\- guest2.img
snap1.img is supposed to be read-only, and there's no guarantee that
/dev/fdset/1 will be available for both guest1 and guest2 to rewrite the
header. Thus, I argue that snap1.img must encode the name base.img in
its metadata, but that we must also have a way to tell qemu to open
/dev/fdset/1 instead of trying open(base.img) in order to get the open
fd for base.img.

I think the -blockdev command is going to solve this.
Stefan, do you have any information on the -blockdev support?

But even though we are still not at a point where the overall feature is
useful, I can at least review these patches for getting us incrementally
closer to our goal.